Foldable or collapsable structures can be folded after they are constructed, transported as a folded construct, and then deployed at a chosen site on demand. They also can be closed, stored and redeployed again if necessary. Although there is no reason according to basic engineering principles why such three-dimensional structures cannot be developed in a variety of forms, only a very few have actually been designed. A major reason for this phenomenon is that such three-dimensional structures must satisfy at least three different kinds of design constraints. First, the structure must have a desirable final and initial geometry (which is determined by the use of the structure); these are termed geometric constraints. Second, the structure must be stable in at least two specified positions--an open, erected state and a closed, compacted state; these are termed kinematic or stabilizing constraints. Third, the structure must be deployable into an erected state without distintegrating in the process or without causing component failure during deployment; these are termed mechanical constraints. Design parameters for making structures which satisfy all three major constraints have proven extremely difficult to formulate. In addition, commercially useful, foldable and deployable three-dimensional structures place additional demands on structural strength, strength to weight ratio, ease of construction, demonstratable mechanical properties in the organizational constituents, flexibility of design to satisfy a wide variety of geometric forms, ease of deployment for the structure in a variety of different environments and attachments for covering membranes, pre-fabricated components and other secondary attachments.
It has been disclosed by U.S. Pat. Nos. 3,968,808; 4,026,313; and 4,290,244, respectively to Zeigler to form deployable structures that are "self-stabilized" by the use of "self-locking" stresses which he claims is necessary for the structure to hold itself in place. These "self-locking" stresses are induced by bending and twisting of certain structural members formed of scissors-like elements. Because some of the structural forms are superficially similar to the present invention of Zeigler and because these known structures share some constituent parts in common with the present invention, a detailed summary of these Zeigler structures is deemed beneficial.
The Zeigler structures generally are a network of scissors-like elements which extend in three dimensions to form, for example, flat structures, semicylindrical structures, semispherical structures and a combination of these. The shell framework is constructed of scissors-like elements formed of two rods which are crossed into pairs, the majority of which are pivotally joined by a pin to form the element resembling scissors. The ends of each scissors-like element are joined into a series of outer apical points and a corresponding series of inner apical points, which define the outer and inner geometric configuration on the structure. A series of crossed pairs of rod elements extend arch-like to form a series of ladders spaced in parallel positions. Each of the inner and outer apical points includes a hub member. Within this structural framework the following features are plainly evident. In the Zeigler structure, when one neglects the stresses caused by gravity, there exist stresses in the deployed structure caused by intentional bending the rods forming the scissors-like elements. The slight bending of the rods keeps the structure stable and produces what is termed cumulative "self-locking stresses". These "self-locking stresses" are said to be necessary for the structure to be self sustaining. Unfortunately, introducing this residual self-locking stress wherein the rods are bent affects the geometry, kinematics and its dynamic behavior. It adversely affects structural strength of the fully deployed structure.
In order to introduce the requisite "self-locking stress" for the structure to be self-supporting in the fully erected state, some of the rod elements are buckled in the fully deployed state. In this manner, redundant stresses are introduced into the structure by bending or actually buckling several of the rod elements into a permanently twisted orientation. The alleged advantage of twisting the structure is said to be two-fold. During deployment, the twisting action is said to force the individual rod elements into their proper places and help the structure "click" or stabilize. After deployment, the twisting action is said to help lock the structure into its final configuration. In point of fact, the buckling and twisting of individual rod elements in the fully deployed state is not necessary. The buckling and twisting greatly reduces the load-carrying capability of the rods and thus the structure as compared to a structure wherein the individual rods are not twisted or bent. The axial load-carrying capability of the rods is drastically reduced by bending it. Zeigler refers to his structures as "self-sustaining" structures. The buckled nature of the rods dramatically reduces the load-bearing capability of his structures. The buckled nature of the rods prevents him from using telescopic elements to replace the rods.
During deployment, some of the forces in the crossed pair of rod elements are so great that the individual rods forming a single scrissors-like unit are forced apart from each other. In some cases to avoid the destructive effects of these buckling forces, either the pivotal connection between the individual rod elements is removed or the pivotal point is purposely designed to permit a free sliding relationship between the individual rod elements. In some cases, complete scissors are removed from the structure.
In U.S. Pat. No. 3,968,808, Zeigler claims that ". . . structural integrity results from a relationship among the rod-like elements which is attained by and incidental to the erected shape itself and which does not rely upon physical constraint as pivotal connections among rod elements." But the nature of the Zeigler geometry forces the introduction of a host of extraneous mechanisms to render the structures of U.S. Pat. No. 4,290,244 practical (see FIGS. 3, 4, 5 and 6). Also, Zeigler states that the rods connected to the central inner apical points of the structure lie in a flat plane. Zeigler provides an entire class of structures which is purposely designed as a series of scissors-like ladders comprising joined rod elements criss-crossing each other into organized ladder formats in which at least two points of each ladder have the crossing rod elements disposed in freely slidable relationship and which there are specific alternating zones of sliding and fixed pivotal crossing points of the rod elements. It will be specifically noted that this class of structures is always less stable in the fully erected state due to the absence of pivotal connections between all individual rod elements.
Overall, therefore, it is apparent that these Zeigler structures contain inherent defects and flaws and place specific limitations on the intended user as regards geometric, kinematic, mechanic and structural strength (bad load-carrying capability). In particular it would be a recognized advantage and major advance for structural frameworks and assemblies to assume a stress-free state in at least the open and closed positions; to avoid use of redundant forces which buckle, bend or twist constituent parts of the structure in the final deployed configuration; to avoid permanent deformation of any structural component; and to provide the user with arbitrarily flexible design parameters such that any geometric configuration and set of dimensions and specifications can be met.